Liquefied carbon dioxide has excellent properties as a solvent and is known to be capable of readily removing particles, oil and other contaminants from a substrate. Wafers used in the semiconductor industry, in particular, require substantially complete removal of the particles adhering even in trace amounts, to the wafer surface.
It has been found that, even commercially available high-purity (99.999%) liquefied carbon dioxide contains a large number of particles. For example, a sample of commercial 99.999% pure liquid carbon dioxide was found to contain 10.sup.8 particles larger than 0.3 micrometer per cubic foot.
If a substrate requiring ultracleaning, such as a semiconductor wafer, is cleaned with dry ice snow obtained from such carbon dioxide, particles are removed from the substrate surface but, at the same time, particles contained in the dry ice snow are deposited in their place. For this reason, it has heretofore been impossible to use such commercially available high-purity liquefied carbon dioxide.
It is general practice to remove particles from liquefied carbon dioxide by means of a filter. However, it has been shown in an experiment in which liquefied high-purity carbon dioxide was passed through a filter in the liquid state that substantially no particles can be removed from liquefied carbon dioxide by means of a filter.